Hi all I have a 1982 F-100 with the 4.9l straight six with 150000 original km's on her.She doesn't use adrop of oil and runs great when on the road but it is missing when it's just idling.I don't think it's an ignition problem as everything checks out, I was going to replace the carb or does anyone think it could be burn't valves? then I would have to have the head redone???
Any suggestions?
thanks

 

probally normal at idle. a few days ago there was a big discussion here about 4.9s "missing "at idle.was finally agreed that it is a lean miss condition at idle.most thought it was cool! a 4.9 with a carb. is even more noticable. there could mabye be a lean code if you pull the codes. yes, it has a pcm and a feedback carb.

 

Here is a quick check for a burnt or otherwise nonsealing exhaust valve. Hold the tip of a rag over the end of the exhaust pipe while the engine is idling. There should be a steady stream of exhaust pushing the rag away from the pipe. However, if every once in a while you see the rag tip get sucked toward the pipe, you have a bad exhaust valve in the engine.

Also, do a compression test on that motor. You could have a blown head gasket between 2 cylinders and one symptom of that is a rough idle.

 

Check and make sure EGR is seating all the way also. I am a big fan of sticking EGRs as I have seen it on too many cars and trucks causing problems. It never hurts to pull it, clean it out and know that isn't a problem for a few more miles. They carbon up quick and once they stop seating correctly, will act like a vacuum leak. Most don't need to be replaced unless the diaphram is is leaking which most aren't. You can just pull it, clean it up with some careful brushing and scraping with lots of WD-40 and the carbon that has built up will go away and let it seat better.

They can cause a miss at idle and then when they get worse, will cause it to want to stall when coming off a highway, pulling up to stop signs etc.

Normal action is to open at higher RPMs and close all the way at low RPM. Once they start sticking open at low RPM, even a hair, weird things happen.

Good Luck,

Lee

 

There can be any number of causes for a miss or lean condition at idle. The 4.9 is notorious for hair line cracks to appear in the intake/exhaust manifold. I have heard one crack for no apparent reason. Yes, I was doing a tune-up on a 1980 4.9 and had just taken my timing light off the #1 cylinder when I heard a "ting" and the engine stumbled. Upon closer inspection, I saw a tiny fracture in the manifold at the second stud mount from the front. I had not done any wrenching on the manifold or carb other than to adjust the curb idle screw. If you suspect the EGR plate, you can remove the vacuum module and make a blank aluminum spacer in place of the gasket (without the exhaust passage cut outs) then reassemble. If the miss does not go away at this point, you need to look deeper into the problem. To check for manifold leaks, I use starting fluid and a tachometer. This is extremely dangerous, but is more reliable than oil or freon to uncover leaks. Note: Freon sucked into the engine causes a serious miss, oil sucked into the engine causes smoke from the exhaust, starting fluid, or butane lighter fluid sucked into the engine causes a rapid increase then drop of idle RPM. Both starting fluid (ether gas) or lighter fluid (butane/benzine gas) are volitile gases that can explode or cause serious burns if used carelessly. The safe way to determine a lean condition is with an exhaust gas analyzer. Most shade tree mechanics do not happen to have one in their tool stash. A silent friend in the emissions control business may be consulted for this test. This test will only indicate the presence of a good or bad air/fuel mixture. A bad reading here, alone with no other tests, will not diagnose the cause. My opinion: If a slight miss at idle is not troublesome, leave it alone and call it a 4.9L Ford thing. When it gets worse it will be more noticable and troublesome, that is the time to start testing for cause.

 

Another "mebie" cause is a cracked spark plug....heat makes it expand, enlarging the slight gap between the "break". Even a plug with a hairline crack around the glass portion is enough for a miss....a loose fitting plug wire, or arcing plug wires against metal or close enough to the next cylinder to cause cross firing arcing. (Noticeable at night while dark) Connect a timing light to each plug to find the miss, if it blinks and stop blinking on that plug wire...you've found your miss.